The present invention relates to charge pumps for use in integrated circuits. More particularly, the present invention relates to a charge pump having a low voltage ripple.
The demand for less expensive, and yet more reliable integrated circuit components for use in communication, imaging and high-quality video applications continues to increase rapidly. As a result, integrated circuit manufacturers are requiring improved performance in the voltage supplies and references for such components and devices to meet the design requirements of such emerging applications.
One device utilized for providing a regulated voltage supply is a charge pump circuit. Charge pumps are DC/DC converters that utilize a capacitor instead of an inductor or transformer for energy storage, and are configured for generating positive or negative voltages from the input voltage. Charge pumps can be configured in various manners, including a charge pump voltage doubler circuit, i.e., a charge pump circuit configured for doubling the input voltage, as well as tripler and inverter configurations. Charge pump circuits can be configured for single phase operation, with a single charge pump capacitor used to charge current during one phase of operation and discharge current during another phase of operation. For example, with reference to FIG. 1, a basic charge pump doubler 100 is illustrated, having a pump capacitor CPUMP, as well as switches S1, S2, S3 and S4 that are configured for single phase operation. In addition, charge pump circuit 100 comprises a reservoir capacitor CRES for maintaining charge to a load device. Basic charge pump doublers exhibit undesirable output voltage ripple due to uncontrolled inrush and charging currents, among other reasons.
To improve voltage ripple, charge pump circuits can also be configured for dual phase regulation, in which two charge pump capacitors are configured to operate during both phases of operation, i.e., one of the capacitors is charging current and the other capacitor is discharging current during each phase of operation. For example, with reference to FIG. 2, a charge pump circuit 200 configured for dual phase voltage operation is illustrated. Charge pump circuit 200 includes a charge pump control circuit comprising four switches S1, S2, S3 and S4 for use with a first pump capacitor CPUMP1 configured to discharge and supply current to a load device during a first phase while a second pump capacitor CPUMP2 is being charged, and four switches S5, S6, S7 and S8 for use with second pump capacitor CPUMP2 configured to discharge and supply current to the load device during a second phase while first pump capacitor CPUMP1 is being charged.
Other approaches to charge pump circuits include the implementation of a latch triggered upper switch drive. For example, with reference to FIG. 3, a charge pump circuit 300 configured as a dual phase doubler comprises a latch triggered upper drive 360 for driving upper switches S3, S4 and S7, S8 in phase with lower switches S1, S2, and S5, S6, wherein nodes 310 and 320 are pulled down to ground on one cycle and up to supply voltage VS+ during the second cycle. Such dual phase charge pump doublers as 200 and 300 have improved output voltage ripple as compared to single phase doublers, but still have significant ripple due to current drawn by charge pump capacitors CPUMP1 and CPUMP2, as well as reservoir capacitor CRES.
Another approach to improve ripple performance includes the use of regulated charge pump circuits having a regulation loop. For example, a dual phase charge pump regulator can comprise a latch triggered upper drive and a regulation loop comprising a regulated voltage VREG and an amplifier. The amplifier is configured to sense the difference in voltage from pump voltage VPUMP and supply voltage VS+, i.e., a difference voltage VREG, to regulate charging of the pump capacitors. Thus, for example, through operation of the regulated feedback loop of the amplifier, a regulated voltage VREG, e.g., 2 volts, is forced between pump voltage VPUMP and supply voltage VS+. Accordingly, regulated charge pump circuits can regulate not only DC, but also during the discharge phase from which ripple occurs.
One problem inherent in any charge pump circuit is the presence of parasitics on the pump capacitors. For example, for charge pump circuit 300, charge pump capacitors CPUMP1 and CPUMP2 include parasitic capacitors 330 and 350 and parasitic capacitors 370 and 390, respectively. Parasitic capacitors 330, 350, 370 and 390 can often have a capacitance between approximately 1% to 10% or more of the capacitance of charge pump capacitors CPUMP1 and CPUMP2, e.g., for 50 pF of capacitance for pump capacitors CPUMP1 and CPUMP2, parasitic capacitors 330, 350, 370 and 390 can comprise approximately 0.5 pF to 5 pF of capacitance. During the phase changes, which occur very rapidly, parasitic capacitors 330, 350, 370 and 390 must also be charged in a very short amount of time. Even with the addition of an amplifier and a regulator loop that is capable of supplying small currents, such a regulator loop will not sufficiently respond to fast transient requirements.
Thus, reservoir capacitor CRES must supply the glitch current, e.g., up to 4 mA or more, required to recharge parasitic capacitors 330, 350, 370 and 390. However, when reservoir capacitor CRES supplies the glitch current di/dt, a voltage drop of up to approximately 150 mV or more can occur at the output of charge pump circuit 300, resulting in undesirable voltage ripple.
Accordingly, a need exists for an improved charge pump regulator that can suitably charge the parasitics of the charge pump capacitors, thus improving voltage ripple performance.
A charge pump circuit is configured for charging of parasitic capacitances associated with charge pump capacitors in a manner that minimizes voltage ripple. In accordance with one aspect of the present invention, the charge pump circuit is suitably configured with an independent charging circuit configured for supplying the current needed to charge the parasitic capacitances, rather than utilizing the reservoir capacitor to supply the needed current. As a result, the total voltage ripple can be reduced, e.g., by a factor of up to ten times or more, during operation of the charge pump circuit. The independent charging circuit can be implemented with various configurations of charge pump circuits, such as single phase or dual phase charge pumps, and/or doubler, tripler or inverter configurations.
The independent charging circuit is configured to minimize introduction of noise to charge pump circuit. In accordance with an exemplary embodiment of the present invention, the independent charging circuit is local to the charge pump circuit for supplying the current for charging the parasitic capacitances, instead of having the current supplied through external bussing or wire bonds.
The independent charging circuit can be configured in various manners for supplying the current required for charging the parasitic capacitances. In accordance with an exemplary embodiment comprising a dual phase charge pump doubler, the independent charging circuit comprises a parasitic charging capacitor configured with a pair of switch devices configured to facilitate charging of the parasitics during both phases of operation of dual phase charge pump circuit. However, the independent charging circuit can comprise any voltage source for providing a charge, as well as fewer or additional switch devices based on the number of pump capacitors.